Even a flu with a low percentage of
lethality can cause a large number of deaths if vast swaths of populations are infected -- seasonal flus kill an estimated 250,000 to 500,000 people worldwide each year.

International health experts, who say the epidemic will spread regardless of attempts at
containment, advise against closing borders, which will not stop the virus but could cause economic collapse and possibly increase the death rate.

But it has shown very little ability to pass from person to person, mainly infecting poultry, and some experts have suggested that there may be something about the H5N1 virus that makes it inherently less
transmissible among people.

This outbreak has caused concern because officials have never seen this particular strain of the flu passing among humans before, said Dr. Anthony S. Fauci, director of the National Institute of Allergy and
Infectious Diseases.

As the virus has continued to spread without causing deaths or even large numbers of
hospitalization, many experts have been questioning whether the new strain of flu is deadlier than normal seasonal flu.

As a benchmark, the deadliest influenza pandemic in the past century, the Spanish influenza of 1918 to 1919, had an estimated
mortality rate of around 2.5 percent but killed tens of millions of people because it spread so widely.

In one sign that the disease may not be as serious as feared, Mexican Health Minister Jose Angel Cordova said that the flu, influenza A(H1N1), appears only slightly more
contagious than the seasonal flu, less than thought.

Unlike typical flu seasons, when infants and the aged are the most
vulnerable, none of the initial deaths in Mexico were in people older than 60 or younger than 3, a spokeswoman with the World Health Organization said.